Electric-field transients covering the extremely wide frequency range from 0.5 to 26 THz are generated in the organic nonlinear crystal 4-N,N-dimethylamino-4'-N'-methylstilbazolium 2,4,6-trimethylbenzenesulfonate (DSTMS). Parametric difference frequency mixing within the spectrum of 25-fs amplified pulses centered at 800 nm provides a highly stable broadband output with an electric-field amplitude of up to several hundred kilovolts/cm. The high stability of the terahertz pulse parameters allows for sensitive phase-resolved broadband spectroscopy of optically thick crystalline samples.

The present work is on the linear and third order nonlinear optical properties of single walled carbon nanotubes functionalized noncovalently with free base porphyrin and a metalloporphyrin separately. The functionalized carbon nanotubes (CNTs) are soluble in dimethylformamide, and the microscopic images show that the porphyrin molecules are attached to the surface of the CNTs. The interaction between CNTs and porphyrins is noncovalent and probably due to π-π interaction as both CNTs and porphyrins have π-electron rich structures. The samples exhibit large ultrafast nonlinear absorption as evident from the open aperture z-scan studies performed using mode-locked Ti:Sapphire femtosecond pulses at 780 nm. The mechanisms of optical nonlinearity of the functionalized CNTs appear to be two photon absorption along with nonlinear scattering with a small contribution from saturable absorption, whereas nonfunctionalized CNTs are known to exhibit saturable absorption. The value of the nonlinear absorption coefficient for the CNTs functionalized with the free base porphyrin is one order smaller than that for CNTs functionalized with metalloporphyrin. Efficient low threshold ultrafast optical limiting is demonstrated for both types of functionalized CNTs.

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